In recent years, in areas of medical treatment and in industrial applications, the use of endoscopes has been broadly adopted. In particular, broad use has been made of endoscopes with improved insertability to positions where observations of objects are made, and because the distance from the observed object to the observation unit is set at a fixed distance for ease of observation, the tip of the insertion part of the endoscope is fitted with an illuminator and an imaging device, as well as with a transparent cover that encompasses the illuminator and the imaging device.
Conventionally, as an endoscope provided with a transparent cover that encompasses an illuminator and an imaging device provided at the tip of the insertion part of the endoscope, arrangements have been proposed as set forth, for example, in Japanese Laid-Open Patent Application 2001-91860 and WIPO Patent Publication WO 01/65995.
Japanese Laid-Open Patent Application 2001-91860 discloses a capsule endoscope, and WIPO Patent Publication WO 01/65995 discloses an imaging device for a capsule endoscope. A cross-sectional view of the capsule endoscope of Japanese Laid-Open Patent Application 2001-91860 is shown in FIG. 12, and a cross-sectional view of the imaging device for a capsule endoscope of Patent Publication WO 01/65995 is shown in FIG. 13.
As shown in FIG. 12, using the notations of Japanese Laid-Open Patent Application 2001-91860, the capsule endoscope thereof includes an objective lens 22 within a hemispheric transparent cover 17 and an image sensor 111 that continuously receives an updated image formed by the objective lens 22 using light provided by light emitting diodes (LEDs) 30, 30 that surround the objective lens 22.
In addition, using the notations of WIPO Patent Publication WO 01/65995, FIG. 13 shows the capsule endoscope thereof that is constructed with an approximately hemispheric transparent cover 21, a CMOS image-forming camera 24 that detects an object image formed by the optical system 22, and illuminators 23, 23 formed of white light LED light sources, or similar light sources, that illuminate the inside of a body cavity that reflects light through the optical system 22 to form the object image on the CMOS image-forming camera 24.
However, with regard to the construction related to the elements noted above of Japanese Laid-Open Patent Application 2001-91860 and WIPO Patent Publication WO 01/65995, with an endoscope in which an illuminator and an objective optical system are provided on the inside of a transparent cover, problems may develop as follows.
When an endoscope constructed as described in Japanese Laid-Open Patent Application 2001-91860 or in WIPO Patent Publication WO 01/65995 is inserted into the body cavity of a subject for the purpose of diagnosis, as the endoscope proceeds within the body cavity, the outer surface (i.e., the surface of the transparent cover against the object part intended to be observed) may become completely covered with various light-dispersing substances, such as digestive fluids, viscous membranes, and so on.
In addition, the transparent cover is made from a relatively soft material in order to prevent breakage of the transparent cover. However, because the cover material is soft, damage of a minute character, such as scratches, may easily occur on the outer surface of the transparent cover.
Because of this, if the illumination light emitted from the endoscope illuminator reaches the outer surface of the transparent cover that has been damaged or covered by light-dispersing substances, the illumination light may be scattered or reflected back to the inside of the transparent cover as what is termed herein as “rearward-dispersed” light. Such rearward-dispersed light, if transmitted through the image-forming optics within the field of view, including through the entrance pupil of the objective optical system, will result in this light interfering with, and distorting, the desired object image. Thus, such rearward-dispersed light is a major obstacle in obtaining the desired observational information.